Across Canada there are 17 different ways of calculating heat warnings. In addition, Nunavut and northern Quebec do not have any heat warning criteria.
Regional heat warnings are issued in Canada as described in the following map.
The variations across the country are set up in such a way that the number of heat warnings come out somewhat evenly. Since the southern interior of BC and southern Ontario experience the hottest summers in Canada, they have the highest bar to be reached before a heat warning is issued.
This first graph lists 33 different places across Canada, covering all 18 zones (including Nunavut), listed from hottest to coldest. Hotness was calculated by averaging the number heat warning days across all 17 thresholds.
Penticton averages 7.1 days per year across all zones, making it the 6th hottest on the list. But using the criteria for the southern interior of BC, Penticton only averages 0.50 days per year.
By contrast, the northern BC city of Terrace averages 0.93 days per year using all 17 criterion, but with the Environment Canada designated threshold, it actually meets the heat warning designation 5.1 days per year.
Northwestern BC and the Yukon have the lowest threshold requirements in Canada while Penticton, Kamloops and the rest of the BC southern interior have the hardest thresholds to meet.
This is not universally true, however, because these southern interior hotspots like Penticton and Kamloops have an easier time meeting their own requirements than those of southwestern Ontario (around Windsor). Penticton and Kamloops average 0.50 and 1.33 heat warning days per year, but if these cities were using Windsor’s thresholds they’d only get 0.03 and 0.10 days per year, respectively.
Of the 17 heat warning different thresholds, Windsor is the national leader in 16 of them. Only with the BC southern interior criteria does Windsor lose out to Kamloops.
By contrast, using the hot and humid southern Ontario criterion yields the following result:
BC’s dry interior climate means cooler nights and fewer days with a humidex, so using Windsor’s criteria puts Penticton (0.03 days) well below more humid cities like Winnipeg (0.43) and Abbotsford (0.13).
The next graph shows this same data with coastal BC thresholds. This reflects the fact that the Fraser Valley, inland from Vancouver, reaches much higher temperatures in the summer.
And here is the same graph using the Atlantic province thresholds.
At the easy end of the scale is the Yukon and northwestern BC. Also included in the following graph is the large area covering central and northeastern BC, northern and central Alberta, and the Northwest Territories.
Notice that using the Yukon criteria would give Windsor more than 45 heat warning days per year. And that’s just an average summer. Some years could be almost every day all summer!
I suppose Environment Canada wanted to at least have a threshold low enough for the Yukon to achieve heat warnings once in a while, but clearly this same benchmark would not make sense for southern Ontario since so many heat warnings would cause the population to tune them out.
The Yukon criteria gives Whitehorse 0.17 days per year, and it even gives Baker Lake, Nunavut 0.03 days per year (although it’s hard make out in the graph). Port Hardy is the only city in the graph that never gets any heat warnings in any category.
The same cannot be said for the northern Northwest Territories community of Inuvik which averages 0.57 days using the NWT thresholds and 0.63 days with the Yukon thresholds.
Quebec is an interesting one because it’s the only place in Canada that doesn’t look at the daily lows. This greatly boosts the southern BC numbers because of the cooler nights relative to Ontario and Quebec.
The remaining heat warning categories are in the following chart. Notice that northern Ontario and southern Saskatchewan are similar, while the other two are similar.
Circling back to each city’s own designated threshold, recall that Penticton only has one heat wave day every two years on average. The limiting factor is the cold nighttime temperature since daytime temperatures are frequently well above 35°C/95°F threshold.
Now, because other places have lower thresholds, the follow locations have more heat wave days per year than Penticton:
- Thunder Bay
- Happy Valley – Goose Bay